It is known to transmit high carrier frequency ultrasonic pulses to a biological tissue and to store the high frequency electric signal received by the transducer after reflection in the tissue. With respect to the transmitted pulse the spectrum of this echo has undergone significant modifications and the Fourier analysis thereof makes it possible to obtain information concerning the structure of the tissue, e.g. for identifying the nature of a tumour, in particular by comparison with the echo spectrums obtained in different tissues or at different echographic observation points. The drawback of this method is the complexity of the apparatus required for carrying out a rapid Fourier analysis and the difficulty of visually interpreting the results.